Quicktime Broadcaster is an extremely powerful broadcasting tool that is similar to FME but for the Mac that you can broadcast in H.264. Let us first go through some of the network settings that you will need that are specific to streaming to Justin.tv.
The "Host Name" will always be the same, live.justin.tv:1935/app. "File" will be live_user_username (where username is your Justin.tv account name in all lowercase). For the dialog's "Username" setting, copy and paste the code from the Advanced Broadcasting page "Stream Key" section. This code will end with a long unique identifier. Make sure you copy and paste it exactly as it is. Lastly, the "Password" is always: jtv and be sure to check "Broadcast over TCP".
Here are some baseline settings for video/audio that might be helpful. You may want to try tweaking the settings as your internet connection allows to raise the quality.